Amplifier neutralizing system



May 17, 1938.

G. W FYLER AMPLIFIER NEUTRALIZING SYSTEM Filed June 22, 1935 g 4 r n mwrO nw m wm I s Patented May 17, 1938 UNITED STATES PATENT OFFICE GeorgeW. Fyler, Schenectady, N. Y., assignor to General Electric Company, acorporation of New York Application June 22, 1935, Serial No. 27,899

6 Claims.

- 10 venting inter-electrode capacity coupling between the output andinput circuits of a push-pull connected amplifier is well known. Suchneutral izing connections usually comprise a neutralizing condenserconnected between the control grid of 5 each tube and the anode of theother tube. It

is desirable in amplifiers of this type to employ neutralizingcondensers of fixed capacity, and this is particularly true in largepower installations where the size of the condensers required 20 wouldmake the cost of variable capacitors prohibitive. However, it has beenfound that if fixed neutralizing condensers be used some method ofmaking minor adjustments must be provided if satisfactory neutralizationis to be obtained. The 25 necessity for such adjustment is due primarilyto variations in the characteristics of the tubes and condensers used inthe construction of such amplifiers. Because of the manufacturingtolerances allowed in the manufacture of such appa- 30 ratus it isdiflicult to obtain tubes and condensers having characteristics whichare identical, and which conform exactly to the characteristicsspecified by the manufacturer. The effect of stray electromagnetic andelectrostatic coupling between the circuit leads in the input and outputcircuits of amplifiers of this type must also be neutralized ifsatisfactory operation is to be ob tained.

In accordance with my invention the advan- 40 tage of using fixedneutralizing condensers is retained and, at the same time, a means isprovided whereby the minor adjustments necessary to obtain substantiallyperfect neutralization may easily be made. 45 It is an object of myinvention to provide in an amplifier of the push-pull connected type animproved inter-electrode capacity coupling neutralizing system whichincludes an inductance in the neutralizing circuits which is adjustableto obtain substantially complete neutralization of the inter-electrodecapacity coupling between the output and input circuits of theamplifier.

It is a further object of my invention to provide in a push-pullconnected amplifier a pair of neutralizing circuits which includeconductors connected respectively in each of the circuits and arrangedin adjustable inductive relation.

More specifically, it is an object of my invention to provide anarrangement of the above type in which portions of the neutralizingcircuit conductors are arranged in inductive relation and mounted on amechanical supporting structure whereby the portions of the conductorsmay be moved relative to one another to change the mutual inductancetherebetween.

The novel features which I believe to be char acteristic of my inventionare set forth with particularity in the appended claims. My inventionitself, however, both as to its organization and method of operationtogether with further objects and advantages thereof will best beunderstood by reference to the following specification taken inconnection with the accompanying drawing in which Fig. 1 shows anamplifier of the push-pull type having my invention embodied therein,and Fig. 2 shows the equivalent bridge circuit of the circuit shown inFig. 1.

Referring to Fig. 1 of the drawing I have shown an amplifier systemconnected in push-pull relation and arranged to be controlled by asource of oscillations, as for example the output from a carrier waveoscillation generator, impressed on the primary winding of thetransformer l and to supply its amplified output to the primary windingof a coupling transformer 2. The seconclary of the transformer I has itstwo ends connected respectively to the control grids of the electrondischarge tubes 3 and 4. The midpoint of the secondary of thetransformer l is connected to a common cathode lead 6 of the tubes 3 and4 through the ground connections indicated at 5. The respective anodesof the tubes 3 and 4 are connected to the two ends of the primarywinding of the transformer 2; which primary has its central pointconnected to the common cathode lead 6 through the source of platepotential 1. The output circuit of the amplifier may be tuned to theoperating carrier frequency by a variable condenser 8. 4

The operation of the above-described amplifier is well understood in theart and only a short explanation thereof is deemed to be necessary.Briefly described, as the oscillating input potential impressed on theprimary winding of the transformer l alternates between positive andnegative values the tubes 3 and 4 alternately supply an amplified outputcurrent to the amplifier output circuit. The outputs from the two tubes.re combined in the tuned output circuit including the primary of thetransformer E. and are impressed on the secondary thereof.

In order to prevent any coupling between the input and output circuitsof the system the neutralizing circuits 9 and it are provided whichinclude respectively the fixed neutralizing condensers H and i2. It willbe noted that the neutralizing circuit 9 is connected between thecontrol grid of the tube 3 and the anode of the tube 4. In like mannerthe neutralizing circuit IE] is connected between the control grid ofthe tube 4 and the anode of the tube 3.

The effect of the two neutralizing circuits is and I is to supplycurrents to the control grids of the two tubes 3 and 4 which are equalin magnitude and opposite in phase to the currents flowing between theinput and output circuits of the amplifier through the couplingtherebetween formed by the capacity existing between the control gridand anode of each of the two tubes. In this manner interelectrodecapacity coupling between the input .and output circuits of theamplifier system is minimized.

It has been found in neutralizing systems of the type described, inwhich fixed neutralizing condensers are employed, that satisfactoryneutralization cannot be obtained unless a suitable means is providedfor making minor adjustments in the neutralizing circuits after theamplifier has been assembled and dining its testing operations. This maybe attributed to the fact that an accurate calculation of theneutralizing current necessary to satisfactory neutralization cannot bemade. Manufacturing discrepancies in the construction of the elementsused in the system, together with stray electrostatic and electromagnetic effects render an exact determination of the magnitude of therequired neutralizing current practically impossible.

In order to overcome the above-described dilficulty I provide a devicefor making minor adjustments which includes a pair of conductors l3 andI4 respectively forming a part of the two neutralizing circuits 9 andit, the two conductors being arranged in inductive relation and suitablymounted for relative movement therebetween. I have shown the conductorsl3 and M as comprising simply portions of the neutralizing leads andhave indicated these conductors as being mounted on a supporting membershown diagrammatically at I5; the conductor l4 being adjustably mountedon the supporting member l5 by means of the two adjusting screws l6 andH, and the conductor l3 being fixedly mounted thereon by means ofinsulators indicated at 58. It will of course be understood that anyother suitable mounting arrangement may be employed and, further, thatthe conductors i3 and I4 may be arranged in any desired manner so longas they are maintained in inductively coupled relation.

With the above arrangement accurate neutralization of the systemdescribed may be obtained by moving the conductors i3 and i4 relative toeach other to change the mutual inductance therebetween. As the mutualinductance is varied the amount of inductive reactance included in eachof the neutralizing circuits is varied, thereby causing a variation inthe value of the effective capacitive reactance in an opposite sense.Thus, if the inductance in each of the neutralizing circuits 9 and H3,is increased, the capacitive reactance is decreased by an equal amountand the total impedance in each of the two circuits thereby beinglowered, the magnitude of the neutralizing currents flowing to thecontrol grids of the tubes is increased. In like manner, if theinductive reactance is decreased by decreasing the mutual effectsbetween the conductors I3 and M, the capacitive reactance in each of thecircuits 8 and If is increased and the magnitude of the neutralizingcurrents flowing to the grids of the tubes 3 and 4 is decreased. It willof course be understood that the value of the mutual inductance betweenthe conductors l3 and I4 and the required adjustment thereof are quitesmall, due to the high operating frequencies and the fairly 'closeapproach to complete neutralization normally obtained in a properlydesigned system. In this connection it will be appreciated thatneutralizing condensers I! and I2 having a capacity reactance valueslightly higher than the calculated value should be used. This isnecessary since the reactance of the coupled conductors l3 and I4 issubstantially purely inductive and an increase or decrease in the valuethereof simply determines the amount by which the capacity reactance ofthe condensers H and I2 is decreased.

The advantages of my improved neutralizing arrangement will be apparentfrom a consideration of Fig. 2 wherein I have shown the equivalentbridge circuit for the system shown in Fig. 1 and described above. Inthe bridge circuit of Fig. 2 the secondary of the input transformer land the primary of the output transformer 2 are shown as being connectedbetween the junction points of the equivalent bridge formed by theamplifier network. The equivalent grid-anode capacity of the tube 3 isrepresented by the condenser l9 and that of the tube 4 by the condenser20. The neutralizing condensers I! and I2 are included in the opposedarms of the bridge and the conductors l3 and M, arranged in inductiverelation, are in series with these two condensers.

It will be observed from an inspection of the equivalent bridge circuitthat in the absence of the variable inductance formed by the inductivelycoupled conductors l3 and [4, the magnitude of the neutralizing currentswould be fixedly determined by the value of the fixed condensers.However, by the introduction in the circuits of the adjustable inductivereactance formed by the inductively coupled conductors l3 and M theeffective capacitive reactance in each of the neutraliz ing branches ofthe bridge may be varied until it is equal to the inter-electrodecapacity of the tube being neutralized. Thus, if the interelectrodecapacities represented by the condensers l9 and 2 are actually lowerthan their rated.

values and the actual capacity of each of the condensers I! and I2 ishigher than the specified rating, the difference may be removed byintroducing inductive reactance in the neutralizing circuits suificientto cancel the required amount of capacitive reactance therefrom,

My improved neutralizing arrangement is particularly useful in thoseinstallations wherein it is desired that the amplifier shall operate atany one of several different frequencies. It has been found that whenthe frequency of the potential impressed on the input circuit of theamplifier is changed, a re-adjustment of the neutralizing system isnecessary if exact neutralization of inter-electrode coupling is to beobtained. By the provision of the adjustable inductances formed by theconductors l3 and M the neutralization may be varied to obtainsatisfactory operation over a wide band of operating frequencies.

My improved neutralizing system may be simply and economicallyconstructed and easily adjusted. Thus, the only element added to theamplifier structure is the mechanical supporting structure formaintaining the portions I3 and I4 of the neutralizing conductors in theproper physical relation.

While I have shown a particular embodiment of my invention, it will ofcourse be understood that I do not wish to be limited thereto since manymodifications in the circuit may be made, and I contemplate by theappended claims to cover all such modifications as fall within the truespirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates, is:

1. An amplifier comprising a pair of electron discharge tubes eachincluding an anode and a control grid, push-pull input and outputcircuits therefor having capacity therebetween requiring to beneutralized, a pair of neutralizing circuits each connected between agrid of one tube and an anode of the other tube, said neutralizingcircuits each including a neutralizing capacitance means providingapproximately the reactance value required for said neutralization, apair of conductors connected respectively in each of said neutralizingcircuits, said conductors being arranged in adjustable inductiverelation, and means whereby the inductance included in each of saidneutralizing circuits may be varied sufficiently to produce completeneutralization of said capacity.

2. An amplifier comprising a pair of electron discharge tubes eachincluding an anode and a control grid, push-pull input and outputcircuits therefor having capacity therebetween requiring to beneutralized, a pair of neutralizing circuits each connected between agrid of one tube and an anode of the other tube, neutralizingcapacitance means in said neutralizing circuits providing approximatelythe reactance required for said neutralization, each of saidneutralizing circuits including a conductor, said conductors beinginductively coupled, and means for varying the coupling between saidconductors sufficiently to produce complete neutralization of saidcapacity.

3. An amplifier comprising a pair of electron discharge tubes eachincluding an anode and a control grid, push-pull input and outputcircuits therefor having inter-electrode capacity requiring to beneutralized, a pair of inter-electrode capacity neutralizing circuitseach connected between a grid of one tube and an anode of the othertube, neutralizing capacitance means in said neutralizing circuitsproviding approximately the reactance required for said neutralization,each of said neutralizing circuits including a conductor, saidconductors having portions thereof inductively coupled, and means forvarying the coupling between said portions sufliciently to producecomplete neutralization of said capacity.

4. An amplifier comprising a pair of electron discharge tubes eachincluding an anode and a control grid, push-pull input and outputcircuits therefor having inter-electrode capacity requiring to beneutralized, a pair of inter-electrode capacity neutralizing circuitseach connected between a grid of one tube and an anode of the othertube, each of said neutralizing circuits including a conductor and aneutralizing condenser, said condensers providing approximately thereactance required for said neutralization, said conductors havingportions thereof inductively coupled, and means for varying the couplingbetween said conductor portions sufiiciently to produce completeneutralization of said capacity.

5. An amplifier comprising a pair of electron discharge tubes eachincluding an anode and a control grid, push-pull input and outputcircuits therefor, and means for neutralizing the effect of theinter-electrode coupling between said input and output circuits, saidmeans including a pair of inter-electrode capacity neutralizing circuitseach connected between a grid of one tube and an anode of the othertube, said last named circuits each including a conductor and acondenser, said condensers providing approximately the reactancerequired for said neutralization, said conductors being inductivelycoupled, and means for varying the coupling between said conductorssufficiently to neutralize completely said effect.

6. An amplifier comprising a pair of electron discharge tubes eachincluding an anode and a control grid, push-pull input and outputcircuits therefor having capacity therebetween requiring to beneutralized, a pair of inter-electrode capacity neutralizing circuitseach connected between a grid of one tube and an anode of the othertube, each of said neutralizing circuits including a conductor and acondenser, said condenser providing approximately the reactance requiredfor said neutralization, said conductors having portions directlyinductively coupled, and means to move a portion of one of saidconductors with respect to the corresponding portion of the otherconductor sufiiciently to produce complete neutralization of saidcapacity.

GEORGE W. FYLER.

